Method of manufacturing semiconductor devices

ABSTRACT

Subjected to obtain a crystalline TFT which simultaneously prevents increase of OFF current and deterioration of ON current. A gate electrode of a crystalline TFT is comprised of a first gate electrode and a second gate electrode formed in contact with the first gate electrode and a gate insulating film. LDD region is formed by using the first gate electrode as a mask, and a source region and a drain region are formed by using the second gate electrode as a mask. By removing a portion of the second gate electrode, a structure in which a region where LDD region and the second gate electrode overlap with a gate insulating film interposed therebetween, and a region where LDD region and the second gate electrode do not overlap, is obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having a circuitcomprising thin-film transistors on a substrate with an insulatingsurface and to a method of manufacture thereof. More specifically, theinvention relates to a constitution of an electrooptical devicerepresented by a liquid crystal display and of an electronic apparatusmounting the electrooptical device. In this specification, thesemiconductor device generally denotes devices that function byutilizing semiconductor characteristics and includes the electroopticaldevices and the electronic apparatus mounting the electroopticaldevices.

2. Related Art

The thin-film transistor (hereinafter abbreviated to TFT) can befabricated on a transparent glass substrate and therefore has beenactively applied to active matrix liquid crystal displays. Because theTFT formed of a semiconductor layer having a crystal constitution(hereinafter referred to as a crystalline TFT) has a high mobility, highresolution image display can be realized by integrating functionalcircuits on the same substrate.

In this specification, the semiconductor film having the crystalconstitution includes a single crystal semiconductor, a polycrystallinesemiconductor, and a microcrystal semiconductor. It also includessemiconductors disclosed in official gazettes JP-A7-130652, JP-A8-78329,JP-A10-135468, and JP-A 10-135469.

To construct an active matrix liquid crystal display, the pixel areaalone requires 1 to 2 million crystalline TFTs and, when peripheralfunctional circuits are constitution, additional numbers of crystallineTFTs are necessary. To operate the liquid crystal display stablyrequires securing reliability of individual crystalline TFTs.

The characteristic of a field-effect transistor such as TFT can beconstitution as comprising a linear region where a drain currentincreases in proportion to a drain voltage, a saturation region wherethe drain current saturates even if the drain voltage increases, and acut-off region where the current ideally does not flow even when thedrain voltage is applied. In this specification, the linear region andthe saturation region are called ON regions of TFT and the cut-offregion is called an OFF region. The drain current in the ON region iscalled an ON current and the current in the OFF region an OFF current.

The pixel area of the active matrix liquid crystal display comprisesn-channel TFTs (hereinafter referred to as pixel TFTs) and, because agate voltage with an amplitude of about 15-20 V is applied thereto,needs to satisfy both of the characteristics of the ON region and theOFF region. On the other hand, the peripheral circuits for driving thepixel area basically comprise CMOS circuits and their characteristics ofthe ON regions are mainly important. The crystalline TFTs, however, havea problem that the OFF current easily increases. Further, a degradationphenomenon has often been observed when driving the crystalline TFT foran extended period of time results in the mobility and ON currentdecrease and the OFF current increases. One of the possible causes ispresumed to be a hot carrier injection phenomenon caused by a highelectric field near the drain.

In the field of the MOS transistor, a lightly doped drain (LDD)constitution has been known as a method for reducing the OFF current andalleviating the high electric field near the drain. This constitutionhas a low concentration impurity region on the inner side of the sourceand drain regions, i.e., on the side of the channel forming region. Thislow concentration impurity region is called an LDD region.

The LDD constitution is also known to be formed similarly in thecrystalline TFT. For example, with the gate electrode as a mask, a firstimpurity injection process forms a low concentration impurity regionthat will form an LDD region, and then sidewalls are formed on bothsides of the gate electrode by using an anisotropic etching technique.With the gate electrode and the sidewalls as a mask, a second impurityinjection process forms a high concentration impurity region that willform a source region and a drain region.

Although the LDD constitution can render the OFF current lower ascompared with that of the ordinary constitution TFT, the LDDconstitution has a drawback that because a series resistance componentincreases for the constitution reason, the TFT ON current, too, isreduced. Further, the deterioration of the ON current cannot beprevented completely. To compensate for this drawback a constitution hasbeen known to be used in which the LDD region overlaps the gateelectrode with the gate insulating film interposed therebetween. Thereare several methods for making this constitution, such as GOLD(Gate-drain Overlapped LDD) and LATID (Large-tilt-angle implanteddrain). In this overlapping constitution the high electric field nearthe drain can be alleviated to increase the tolerance for hot carriersand at the same time prevent reduction in the ON current.

It is also confirmed that in the crystalline TFT, too, the provision ofthe LDD constitution increases with standability against hot carriers ascompared with that of the simple constitution TFT having only the sourceregion, drain region and channel forming region, and that the use of theGOLD constitution produces an excellent effect. This fact is describedin “A Novel Self-aligned Gate-overlapped LDD Poly-Si TFT with HighReliability and Performance,” by Mutsuko Hatano, Hajime Akimoto andTakeshi Sakai, IEDM97-523.

In the crystalline TFT, forming the LDD constitution has proved to be aneffective means for suppressing the hot carrier injection phenomenon.Further, the use of the GOLD constitution is found to be able to preventa reduction in the ON current, which was observed in the LDDconstitution, and also to produce a preferable result in terms ofreliability.

Although the GOLD constitution can prevent degradation of the ONcurrent, it has a drawback that when a high gate voltage is appliedthereto at the OFF characteristic, the OFF current is increased, asobserved in the pixel TFT in particular. An increase in the OFF currentin the pixel TFT produces undesired effects such as increased powerconsumption and abnormal image display. This is constitution due to aninverted layer which, because of the characteristic of the OFF region,is formed in the LDD region overlapping the gate electrode.

As described above, to realize a high level of reliability in thecrystalline TFT requires a comprehensive study on the constitution ofthe device. For this reason, it has been desired that the GOLDconstitution be formed. With the conventional method, however, althoughthe LDD region can be formed self-aligningly, the process of forming thesidewall film by an anisotropic etching is not suited for processing alarge-area glass substrate like the one used in the liquid crystaldisplay. Further, because the length of the LDD region is determined bythe width of the sidewall, the degree of freedom in designing devicedimensions is severely restricted.

If, as in the pixel TFT, the characteristics of both the ON region andthe OFF region are important and an attempt is made to meet the requiredlevel of their reliability and prevent an increase in the OFF current,the conventional GOLD constitution is not sufficient.

SUMMARY OF THE INVENTION

A first object of the invention is to provide a crystalline TFT having aconstitution in which a gate electrode and an LDD region are overlappedby a simpler method than conventional ones, and to provide a method ofmanufacturing such a crystalline TFT. The GOLD constitution has aproblem that when a high gate voltage is applied thereto at the OFFcharacteristic, the OFF current is increased. It is therefore a secondobject of the invention to provide a constitution capable of preventingan increase in the OFF current and a method of manufacturing such aconstitution.

For a pixel area of the liquid crystal display and drive circuits madebasically of CMOS circuits arranged around the pixel area, it is a thirdobject of the invention to provide a constitution in which the LDDregion of at least an n-channel TFT overlaps a gate electrode and whichcan prevent an increase in the OFF current, and to provide a method ofmanufacturing the constitution.

FIG. 17 schematically shows the constitution of a TFT and its Vg-Id(gate voltage-drain current) characteristic, based on the findingsobtained so far. FIG. 17(A-1) shows the simplest constitution of TFT inwhich the semiconductor layer comprises a channel forming region, asource region and a drain region. In the characteristic shown in FIG.17(B-1), +Vg side represent the ON region of TFT and −Vg side an OFFregion. A solid line represents an initial characteristic and a dashedline a characteristic after application of a bias stress. In thisconstitution, the ON current and the OFF current are both high and thedegradation due to the bias stress is large, so that this constitutioncannot be applied to the pixel TFT.

FIG. 17(A-2) shows a constitution in which a low concentration impurityregion that will make an LDD region is added to the (A-1) constitutionin such a way that the gate electrode does not overlap the LDD region.In this constitution, although it is possible to suppress the OFFcurrent to some extent, the degradation of the ON current cannot beprevented, as shown in (B-2). FIG. 17(A-3) shows a constitution, alsocalled a GOLD constitution, in which the LDD region completely overlapsthe gate electrode. As shown in (B-3), the degradation of the ON currentcan be suppressed but there is a problem that the OFF current is higherthan that of the (A-2) LDD constitution.

Hence, the constitution shown in FIGS. 17(A-1), (A-2), and (A-3) cannotsimultaneously satisfy both of the ON region characteristic and the OFFregion characteristic required by the pixel area, within the reliabilityrequirement. It has been found, however, that the use of theconstitution of FIG. 17(A-4) can prevent deterioration of the ON currentand suppress an increase in the OFF current. This can be achieved bydividing the LDD region into a region that overlaps the gate electrodeand a region that does not. The LDD region overlapping the gateelectrode has a function of suppressing a hot carrier injectionphenomenon and the LDD region not overlapping the gate electrode has afunction of preventing an increase in the OFF current.

In the present invention, a gate electrode is formed of a plurality oflayers in order to obtain a constitution in which the LDD regionoverlaps the gate electrode, by a step of forming a first conductivelayer of the gate electrode and a step of forming a second conductivelayer of the gate electrode. After the first conductive layer formingstep, the invention performs a step of first impurity element doping toform a first impurity region that will make the LDD region and, afterthe second conductive layer forming step, performs a step of a secondimpurity element doping to form a second impurity region that will makea source region and a drain region. Then, a part of the secondconductive layer is removed to form a TFT which has a region where theLDD region does not overlap the second conductive layer.

Therefore, the constitution of this invention disclosed in thisspecification is characterized in that, in a semiconductor device havinga pixel area where an n-channel thin-film transistor is provided in eachpixel, the gate electrode of the n-channel thin-film transistor has afirst conductive layer formed in contact with a gate insulating film anda second conductive layer formed in contact with the first conductivelayer and the gate insulating film; and that the semiconductor layer ofthe n-channel thin-film transistor has a channel forming region, a firstimpurity region of one conductivity type formed in contact with thechannel forming region, and a second impurity region of one conductivitytype formed in contact with the first impurity region, the firstimpurity region having a part thereof overlapping a region of the secondconductive layer that is in contact with the gate insulating film.

Another constitution of this invention is characterized in that, in asemiconductor device including a CMOS circuit having an n-channelthin-film transistor and a p-channel thin-film transistor, the gateelectrode of the n-channel thin-film transistor has a first conductivelayer formed in contact with a gate insulating film and a secondconductive layer formed in contact with the first conductive layer andthe gate insulating film; and that the semiconductor layer of then-channel thin-film transistor has a channel forming region, a firstimpurity region of one conductivity type formed in contact with thechannel forming region, and a second impurity region of one conductivitytype formed in contact with the first impurity region, the firstimpurity region having a part thereof overlapping a region of the secondconductive layer that is in contact with the gate insulating film.

Still another constitution of this invention is characterized in that,in a semiconductor device which includes a pixel area where an n-channelthin-film transistor is provided in each pixel and a CMOS circuit havingan n-channel thin-film transistor and a p-channel thin-film transistor,the gate electrode of the n-channel thin-film transistor has a firstconductive layer formed in contact with a gate insulating film and asecond conductive layer formed in contact with the first conductivelayer and the gate insulating film; and that the semiconductor layer ofthe n-channel thin-film transistor has a channel forming region, a firstimpurity region of one conductivity type formed in contact with thechannel forming region, and a second impurity region of one conductivitytype formed in contact with the first impurity region, the firstimpurity region having a part thereof overlapping a region of the secondconductive layer that is in contact with the gate insulating film.

In the constitution of this invention described above, the firstimpurity region forms the LDD region and the second impurity regionforms the source region or drain region. The gate electrode of thep-channel thin-film transistor comprises a first conductive layer formedin contact with a gate insulating film and a second conductive layerformed in contact with the first conductive layer and the gateinsulating film. The semiconductor layer of the p-channel thin-filmtransistor comprises a channel forming region and a third impurityregion of a conductivity type reverse to one conductivity type formed incontact with the channel forming region.

The constitution of this invention described above may comprise asemiconductor layer and a storage capacitance, wherein the semiconductorlayer is provided in contact with the second impurity region and has thesame conductivity type as the first impurity region, and wherein thestorage capacitance is provided by a capacitive line formed by the gateinsulating film, the first conductive layer and the second conductivelayer.

In the constitution of this invention, the first conductive layer isformed of one of elements selected from titanium (Ti), tantalum (Ta),Tungsten (W) and Molybdenum (Mo), or an alloy made mainly of theseelements.

The first conductive layer comprises a conductive layer (A) formed incontact with the gate insulating film and one or more conductive layersformed over the conductive layer (A). It is preferred that theconductive layer (A) formed in contact with the gate insulating film beformed of one element selected from titanium (Ti), tantalum (Ta),tungsten (W) and molybdenum (Mo), or formed of an alloy made mainly ofthese elements, and that at least one of the one or more conductivelayers formed over the conductive layer (A) be formed of one elementselected from aluminum (Al) and copper (Cu) or formed of an alloy mademainly of these elements. The second conductive layer should preferablybe formed of one element selected from titanium (Ti), tantalum (Ta),tungsten (W) and molybdenum (Mo), or formed of an alloy made mainly ofthese elements.

To realize the constitution described above, the method of manufacturinga semiconductor device according to the invention comprises: a firststep of forming a semiconductor layer over a substrate having aninsulating surface; a second step of forming a gate insulating film incontact with the semiconductor layer; a third step of forming a firstconductive layer in contact with the gate insulating film; a fourth stepof forming a first impurity region by adding an element belonging toGroup 15 in the periodic table to the semiconductor layer with the firstconductive layer as a mask; a fifth step of forming a second conductivelayer in contact with the first conductive layer and the gate insulatingfilm; a sixth step of forming a second impurity region by adding anelement belonging to Group 15 in the periodic table to the semiconductorlayer with the second conductive layer as a mask; and a seventh step ofremoving a part of the second conductive layer.

Another manufacturing method of the invention comprises: a first step offorming a first semiconductor layer and a second semiconductor layerover a substrate having an insulating surface; a second step of forminga gate insulating film in contact with the first semiconductor layer andthe second semiconductor layer; a third step of forming a firstconductive layer in contact with the gate insulating film; a fourth stepof forming a first impurity region by adding an element belonging toGroup 15 in the periodic table to at least the first semiconductor layerwith the first conductive layer as a mask; a fifth step of forming asecond conductive layer in contact with the first conductive layer andthe gate insulating film; a sixth step of forming a second impurityregion by adding an element belonging to Group 15 in the periodic tableto at least the first semiconductor layer with the second conductivelayer as a mask; and a seventh step of forming a third impurity regionby adding an element belonging to Group 13 in the periodic table to onlythe second semiconductor layer with the second conductive layer as amask.

Still another manufacturing method of the invention comprises: a firststep of forming a first semiconductor layer and a second semiconductorlayer over a substrate having an insulating surface; a second step offorming a gate insulating film in contact with the first semiconductorlayer and the second semiconductor layer; a third step of forming afirst conductive layer in contact with the gate insulating film; afourth step of forming a first impurity region by adding an elementbelonging to Group 15 in the periodic table to at least the firstsemiconductor layer with the first conductive layer as a mask; a fifthstep of forming a second conductive layer in contact with the firstconductive layer and the gate insulating film; a sixth step of forming asecond impurity region by adding an element belonging to Group 15 in theperiodic table to at least the first semiconductor layer with the secondconductive layer as a mask; a seventh step of removing a part of thesecond conductive layer; and an eighth step of forming a third impurityregion by adding an element belonging to Group 13 in the periodic tableto only the second semiconductor layer with the second conductive layeras a mask.

The manufacturing method of the invention described above ischaracterized in that the first impurity region forms the LDD region andthe second impurity region forms a source region and a drain region. Themanufacturing method further includes: a step of adding an elementbelonging to Group 15 in the periodic table in the same concentration asin the first impurity region to a semiconductor layer extending from thesecond impurity region; and a step of forming a capacitive line by thefirst conductive layer and the second conductive layer.

In the constitution of this invention, the first conductive layer isformed of one element selected from titanium (Ti), tantalum (Ta),tungsten (W) and molybdenum (Mo), or formed of an alloy made mainly ofthese elements.

The invention is further characterized in that the first conductivelayer comprises a conductive layer (A) formed in contact with the gateinsulating film and one or more conductive layers formed over theconductive layer (A); that the conductive layer (A) is formed of oneelement chosen from titanium (Ti), tantalum (Ta), tungsten (W) andmolybdenum (Mo), or formed of an alloy made mainly of these elements;that at least one of the conductive layers formed over the conductivelayer (A) is formed of one element chosen from aluminum (Al) and copper(Cu) or formed of an alloy made mainly of these elements; and that thesecond conductive layer is formed of one element selected from titanium(Ti), tantalum (Ta), tungsten (W) and molybdenum (Mo), or formed of analloy made mainly of these elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a TFT according to one embodiment ofthe invention.

FIG. 2 is a cross sectional view showing a process of manufacturing theTFT.

FIG. 3 is a cross sectional view showing a process of manufacturing theTFT.

FIG. 4 is a cross sectional view showing a process of manufacturing theTFT.

FIG. 5 is a cross sectional view showing a process of manufacturing theTFT.

FIG. 6 is a cross sectional view showing a process of manufacturing theTFT.

FIG. 7 is a perspective view of an active matrix substrate.

FIG. 8 is a top view of a pixel area and CMOS circuits.

FIG. 9 is a schematic diagram showing a process of manufacturing acrystalline silicon film.

FIG. 10 is a schematic diagram showing a process of manufacturing acrystalline silicon film.

FIG. 11 is a schematic diagram showing a process of manufacturing acrystalline silicon film.

FIG. 12 is a schematic diagram showing a process of manufacturing acrystalline silicon film.

FIG. 13 is a cross sectional view showing a process of manufacturing aTFT.

FIG. 14 is a cross sectional view showing a constitution of a gateelectrode.

FIG. 15 is external views showing examples of semiconductor apparatus.

FIG. 16 is a schematic diagram showing the constitution of a gateelectrode.

FIG. 17 is a diagram showing a TFT constitution and an electriccharacteristic.

FIG. 18 is a schematic diagram showing the constitution of a gateelectrode.

FIG. 19 is external views showing examples of semiconductor apparatus.

FIG. 20 is schematic diagrams showing the constitution of a projector.

FIG. 21 is a schematic diagram showing the constitution of an activematrix EL display.

FIG. 22 is a cross sectional view showing the constitution of the pixelarea of an active matrix EL display.

FIG. 23A is a top view showing the constitution of the active matrix ELdisplay.

FIG. 23B is a cross section showing the constitution of the activematrix EL display.

FIG. 24A is a top view showing the constitution of the active matrix ELdisplay.

FIG. 24B is a cross section showing the constitution of the activematrix EL display.

FIG. 25 is a cross section showing a switching TFT and a current controlTFT of an active matrix EL display.

FIG. 26A is a top view showing the constitution of the pixel area of anactive matrix EL display.

FIG. 26B is a circuit diagram of an active matrix EL display.

FIG. 27 is a cross section showing the constitution of an active matrixEL display.

FIG. 28A through 28C are circuit diagrams of an active matrix ELdisplay.

FIG. 29 is a graph showing an example light transmittivitycharacteristic of a ferroelectric mixed liquid crystal.

DETAILED DESCRIPTION OF THE INVENTION

One representative example of the invention will be described byreferring to FIG. 1. Reference number 101 represents a substrate with aninsulating surface. It may be, for example, a glass substrate, stainlesssteel substrate, plastic substrate, ceramic substrate and siliconsubstrate, all with a silicon oxide film. A quartz substrate may also beused.

The surface of the substrate 101 over which a TFT is to be formed isdeposited with a base film 102. The base film 102 is formed of a siliconoxide film or silicon nitride film to prevent an impurity from diffusingfrom the substrate 101 into a semiconductor layer. The base film 102 mayalso be formed of a silicon oxynitride film.

The semiconductor layer formed over the substrate should preferably beformed of a crystalline semiconductor, which is made by crystallizing,through a laser crystallization method or a solid phase epitaxy by heattreatment, an amorphous semiconductor deposited through plasma CVD, lowpressure CVD or sputtering. It is also possible to use amicrocrystalline semiconductor formed by the method described above. Thesemiconductor material that can be used here includes silicon,germanium, silicon-germanium alloy, and silicon carbide. A semiconductorcompound material such as gallium arsenide may also be used.

Alternatively, the semiconductor layer formed over the substrate 101 maybe an SOI (Silicon On Insulators) substrate with a single crystalsilicon layer. There are several kinds of SOI substrates according tothe constitution and the manufacturing method. Typically SIMOX(Separation by Implanted Oxygen), ELTRAN (Epitaxial Layer Transfer:trade name of Canon), substrate, Smart-Cut (trade name of SOITEC) may beused. Other SOI substrates may of course be used.

FIG. 1 shows cross-sectional constitution of n-channel and p-channelTFTs. The gate electrodes of the n- and p-channel TFTs comprise a firstconductive layer and a second conductive layer. In FIG. 1, the firstconductive layer is of a 3-layer constitution, comprising a conductivelayer (A), 111, 115 formed in contact with a gate insulating layer 103,and a conductive layer (B) 112, 116 and a conductive layer (C) 113, 117deposited over the conductive layer (A). The second conductive layer114, 118 is formed in contact with the upper and side surfaces of thefirst conductive layer and extends over the gate insulating layer 103.

The conductive layer (A) 111, 115 which comprise the first conductivelayer is made of Ti, Ta, Mo and W or an alloy consisting mainly of theseelements. The conductive layer (B) 112, 116 is preferably made of Al andCu with a low resistivity. The conductive layer (C) 113, 117 is made ofTi, Ta, Mo and W or an alloy consisting mainly of these elements, aswith the conductive layer (A). The conductive layer (B) is provided forthe purpose of reducing the resistance of the gate electrode,constitution a situation where the TFT of this invention is formed on alarge-area substrate like a liquid crystal display. Depending on theuse, the first conductive layer may be formed of the conductive layer(A) only or of three or more layers deposited one upon the other.

The second conductive layer 114, 118 is electrically conductive to thefirst conductive layer and extends over the gate insulating layer 103.FIG. 16 shows a detailed constitution of the gate electrode. The secondconductive layer is initially formed over a length of L3 and is etchedaway by a length of L5 to have the final length of L2. Hence, if thefirst conductive layer has a length of L1, the distance over which thesecond conductive layer extends over the gate insulating layer 103 canbe expressed as L4.

In this invention, the gate electrodes should preferably be formed sothat L1 is 0.1 to 10 μm and L3 0.5 to 22 μm. The distance L5 by whichthe second conductive layer is etched away needs to be 0.1 to 3 μm.Hence, L2 is 0.3 to 16 μm and L4 0.1 to 3 μm.

The first conductive layer and the second conductive layer also have afunction of mask in the first impurity doping process and the secondimpurity doping process. This fact should be taken into account indetermining the lengths of L1 and L3 and of L2 and L5. As shown in FIG.16, the reason for forming the second conductive layer at first over thelength L3 and then etching away by the length L5 to have the finallength of L2 is to ensure that the area in which the first impurityregion 1605 that will make the LDD region contacts the second gateinsulating film through the gate insulating film has the length of L4and the area in which it does not contact the second gate insulatingfilm has the length of L5 in order to realize the constitution of thisinvention.

In the case where this invention is applied to the pixel area, torealize a practical aperture ratio requires using a low resistancematerial with a thin film resistivity of 2-3 μΩ-cm for the gateelectrode, and thus Al and Cu should preferably be used. Constitutionheat resistance, the gate electrode should preferably have a cladconstitution in which the conductive layer (B) of a low resistancematerial is enclosed by the conductive layer (A) or (C) of the firstconductive layer of the gate electrode which is made of a high meltingpoint metal such as Ta, W and Mo or their alloy and further enclosed bythe second conductive layer.

The semiconductor layer of the n-channel TFT comprises: a channelforming region 104; a first impurity region 105 formed in contact withthe channel forming region; and a source region 106 and a drain region107, both formed in contact with the first impurity region 105. Thefirst impurity region 105 is formed to overlap the second conductivelayer 114 with the gate insulating layer 103 interposed therebetween.

The first impurity region 105 has an n-type impurity elementconcentration of 1×10¹⁶ to 1×10¹⁹ atoms/cm³, typically 1×10¹⁷ to 5×10¹⁸atoms/cm³. The source region 106 and the drain region 107 have animpurity concentration of 1×10²⁰ to 1×10²¹ atoms/cm³, typically 1×10²⁰to 5×10²¹ atoms/cm³.

At this time, the channel forming region 104 may be injected with boronin a concentration of 1×10¹⁶ to 5×10¹⁸ atoms/cm³ in advance. Boron isadded in order to control a threshold voltage and any other elements canbe used if they can produce the similar effect.

Third impurity regions 109, 110 of p-channel TFT are used to form asource region and a drain region. The third impurity regions 109, 110contain an n-type impurity element at the same concentration level asthe source region 106 and the drain region 107 of the n-channel TFT andare also doped with a p-type impurity element in a concentration 1.5 to3 times that of the n-type impurity element. The third impurity regions109, 110 are formed outside the second conductive layer 118 of the gateelectrode.

As described above, the TFT of this invention has a constitution inwhich the gate electrode comprises the first conductive layer and thesecond conductive layer with the second conductive layer formed incontact with the first conductive layer and the gate insulating film, asshown in FIG. 1. The TFT of the invention is also characterized in that,in at least the n-channel TFT, a part of the first impurity regionformed in the semiconductor layer with the gate insulating filminterposed therebetween overlaps the second conductive layer.

The constitution shown in FIG. 1 can be realized by forming the firstimpurity region that will make the LDD region with the first conductivelayer as a mask, and a second impurity region that will make the sourceand drain regions, with the second conductive layer as a mask, and thenetching back the second conductive layer. Hence, the length of the LDDregion is determined by the length L1 of the first conductive layer andthe length L3 of the second conductive layer, and the length over whichthe LDD region does not overlap the second conductive layer can bearbitrarily determined by the length L5 by which the second conductivelayer is etched back. This method is very effective because it canbroaden the degree of freedom in the design and manufacture of TFT.

The p-channel TFT is formed with the third impurity regions 109, 110 butno region for LDD constitution. The third impurity region forms a sourceregion 109 and a drain region 110. Although the LDD constitution of thisinvention may be provided, it is preferred that a characteristic balancebetween the p-channel TFT and the n-channel TFT be achieved by gainingthe ON current because the p-channel TFT inherently has highreliability. When this invention is applied to the CMOS circuit as shownin FIG. 1, this characteristic balance matters very much. It is alsonoted that the constitution of this invention can be applied to thep-channel TFT without any problem.

After the n-channel TFT and p-channel TFT are completed as describedabove, they are covered with a first interlayer insulating film 119 andformed with source lines 120, 121 and a drain line 122. The source anddrain lines are connected to the second and third impurity regionsthrough contact holes formed in the first interlayer insulating film. Inthe constitution of FIG. 1, after these lines are formed, they arecovered with a silicon nitride film as a passivation film 123 andfurther with a second interlayer insulation film 124 of organic resinmaterial. The second interlayer insulation film 124 is not limited tothe organic resin material but, when applied to a liquid crystaldisplay, it should preferably use the organic resin material to secure aplanar surface.

While FIG. 1 shows an example of a CMOS circuit having an n-channel TFTand a p-channel TFT in a complementary combination, it is also possibleto apply this invention to an NMOS circuit comprising an n-channel TFTor to the pixel area of a liquid crystal display.

The constitution of this invention described above will be explained inmore detail in conjunction with the following examples.

EXAMPLE 1

This example describes a constitution of this invention, a method ofmanufacturing simultaneously a pixel area and CMOS circuits that arebasic configuration of driving circuits provided around the pixel area.

In FIG. 2, a substrate 201 is an alkali-free glass substrate,represented, for example, by 1737 glass substrate of Corning make. Thesurface of the substrate 201 where the TFT is to be formed is depositedwith a base film 202 made mainly of silicon oxide to a thickness of 200nm. The base film 202 may use a silicon nitride film or siliconoxynitride film.

The base film 202 may be formed in a single layer of the material aboveor in two or more layers. In either case, it is formed to a thickness ofabout 100 to 300 nm. For example, the base film 202 may be formed by aplasma CVD method as a two-layer constitution, in which a first siliconoxynitride film is formed from SiH₄, NH₃ and N₂O to a thickness of 10 to100 nm and a second silicon oxynitride film from SiH₄ and N₂O to athickness of 100 to 200 nm.

Next, over this base film 202 an amorphous silicon film is deposited bya plasma CVD method to a thickness of 50 nm. The amorphous silicon filmshould preferably be heated at 400 to 500° C. for dehydrogenationdepending on the amount of hydrogen contained to keep the hydrogencontent at 5 atm % or less to ensure a desirable condition forcrystallization.

The process of crystallizing the amorphous silicon film can use a knownlaser crystallization technique or a thermal crystallization technique.In this example, a crystalline silicon film was obtained by focusing aKrF excimer laser beam of pulse oscillation type into a linear beam andapplying it to the amorphous silicon film.

While this example uses an amorphous silicon film as the initial film,it is also possible to use a microcrystal silicon film as the initialfilm or directly form a silicon film having crystallinity.

The crystalline silicon film thus formed is patterned to form islandsemiconductor layers 204, 205, 206.

Next, the semiconductor layers 204, 205, 206 are covered with a gateinsulating film 203 made mainly of silicon oxide or silicon nitride.Here a silicon oxynitride film was deposited by a plasma CVD method to athickness of 100 nm. Then, though not shown, the surface of the gateinsulating film 203 is sputtered with Ta to a thickness of 10 to 200 nm,for example 50 nm, for a conductive layer (A) and further with Al to athickness of 100 to 1,000 nm, for example, 200 nm, for a conductivelayer (B), these conductive layers (A) and (B) together comprise thefirst conductive layer of the gate electrode. Then, a known patterningtechnique is used to form the conductive layers (A) 207, 208, 209 and210 and the conductive layers (B) 212, 213, 214 and 215. At this time,as shown in FIG. 16, the length L1 of the first conductive layer of thegate electrode can be set arbitrarily in the range of 0.1 to 10 μm. Herethe first conductive layer was patterned with L1 set to 2 μm.

When Al is used for the conductive layer (B) making up the firstconductive layer, pure Al may be used or Al alloy may be used whichcontains 0.1 to 5 atm % of one of elements Ti, Si and Sc. When copper isused, it is preferred that, though not shown, a silicon nitride film bedeposited over the surface of the gate insulating film 203 to athickness of 30 to 100 nm.

In FIG. 2, a storage capacitance is provided on the drain side of then-channel TFT making up the pixel area. Line electrodes 211, 216 ofstorage capacitance are formed of the same material as the firstconductive layer.

With the constitution of FIG. 2(A) completed in this way, a firstprocess of adding an n-type impurity is performed to form a firstimpurity region. Impurity elements known to give an n type conductivityto the crystalline semiconductor material include phosphorus (P),arsenic (As) and antimony (Sb). Here phosphorus was used and an iondoping method using phosphine (PH₃) was performed. In this process theacceleration voltage was set at a relatively high level of 80 keV toinject phosphorus through the gate insulating film 203 into thesemiconductor layer below. The impurity region thus formed will makefirst impurity regions 229, 236 and 240 of n-channel TFT described laterwhich function as the LDD regions. Hence, the phosphorus concentrationin these regions is preferably set in the range of 1×10¹⁶ to 1×10¹⁹atoms/cm³. Here 1×10¹⁸ atoms/cm³ was used (FIG. 2(B)).

The impurity elements described above which were doped in thesemiconductor layer had to be activated by laser annealing or heattreatment. This process may be performed after the impurity dopingprocess for forming the source and drain regions. It was, however,effective to activate the impurity elements at this stage by the laserannealing.

In this process, the conductive layers (A) 207, 208, 209 and 210 and theconductive layers (B) 212, 213, 214 and 215, those comprise the firstconductive layer, function as a mask for the injection of phosphorus. Asa result, no or almost no phosphorus was injected in areas immediatelybelow the first conductive layer, with the gate insulating filminterposed therebetween. Then, as shown in FIG. 2(B), impurity regions218, 219, 220, 221 and 222 doped with phosphorus were formed. In thisprocess, a resist mask 217 was formed to prevent the semiconductor layer205 of p-channel TFT forming the CMOS circuit from being injected withphosphorus.

After the resist mask 217 was removed, the second conductive layer ofthe gate electrode was formed. Here, Ta was used as a material for thesecond conductive layer and deposited to a thickness of 100 to 1,000 nm,for example 200 nm. It was then patterned by a known technique to formthe second conductive layers 243, 244, 245 and 246. At this time, thelength L3 of the second conductive layer (corresponding to the lengthrepresented by the same reference number in FIG. 16) was set in therange of 0.5 to 22 μm. Here it was patterned so that the length was 5μm. As a result, the second conductive layer has, on both sides of thefirst conductive layer, regions (L6) 1.5 μm long each over which itcontacts the gate insulating film.

A storage capacitance is provided on the drain side of the pixel TFT,and an electrode 247 of the storage capacitance is formed simultaneouslywith the second conductive layer.

With the second conductive layers 243, 244, 245 and 246 as a mask, ann-type impurity element was added for the second time to form secondimpurity regions. At this time, the resist masks 223, 224, 225, 226 and227 used to pattern the second conductive layer may be left as they are.Here, an ion doping method using phosphine (PH₃) was similarlyperformed. In this process, too, the acceleration voltage was set at arelatively high level of 80 keV to inject phosphorus through the gateinsulating film 203 into the semiconductor layer below. The secondimpurity regions should preferably have a phosphorus concentration inthe range of 1×20²⁰ to 1×10²¹ atoms/cm³ to form source regions 230 and237 and drain regions 231 and 241 of the n-channel TFT. Here theconcentration was set at 1×10²⁰ atoms/cm³ (FIG. 2(C)).

Further, though not shown, it is also possible to remove the gateinsulating film covering the source regions 230 and 237 and the drainregions 231 and 241 to expose these regions of the semiconductor layerand directly inject phosphorus into these regions. The use of thisprocess can reduce the acceleration voltage of the ion doping methoddown to 10 keV and allows for efficient doping of phosphorus.

The regions 233 and 234 of the p-channel TFT are also doped with thesame concentration of phosphorus but because in a later process boron isadded at two times that concentration, the conductivity type is notreversed; posing no problem to the operation of the p-channel TFT.

After the state of FIG. 2(C) was obtained, the resist masks 223, 224,225, 226 and 227 were removed and a photoresist film was formed againand patterned by exposure from the back. At this time, with the firstand second conductive layers used as masks, resist masks 248, 249, 250,256 and 257 were self-aligningly formed as shown in FIG. 3(A). Theexposure from the back utilizes direct light and scattered light, andoverexposure allowed the resist masks to be formed inside the secondconductive layer.

Unmasked portions of the second conductive layer were etched away by anordinary dry etching technique using CF₄ and O₂ gases. Then, as shown inFIG. 3(B), the second conductive layer was etched back by a length L5(corresponding to a portion represented by the same reference number inFIG. 16). The length L5 should be adjusted in the range of 0.1 to 3 μm.Here the length was set to 0.5 μm. As a result, in the first impurityregion of the n-channel TFT that will make the LDD region and which is1.5 μm long, there were formed a region 1 μm long (L4) over which thefirst impurity region overlaps the second conductive layer and a region0.5 μm long (L5) over which the first impurity region does not overlapthe second conductive layer.

Next, with the photoresist masks 258, 259 covering the areas where then-channel TFT is to be formed, a p-type impurity is doped only in aregion where the p-channel TFT is to be formed. The impurity elementsknown to produce a p-type conductivity include boron (B), aluminum (Al)and gallium (Ga). Here boron was used as an impurity element and the iondoping method using diborane (B₂H₆) was performed. The accelerationvoltage was similarly set at 80 keV to add boron at the density of2×10²⁰ atoms/cm³. As shown in FIG. 3(B), third impurity regions 262 and261 heavily doped with boron were formed. The third impurity regions ata later process form a source region 261 and a drain region 262 of ap-channel TFT.

The n- or p-type impurity elements that were added at their respectiveconcentrations are, in their own states, not active and thus must beactivated. This can be achieved by a thermal annealing using electricfurnace, a laser annealing using the excimer laser, and a rapid thermalannealing (RTA) using a halogen lamp.

The thermal annealing activated the impurity elements by heating them at550° C. for two hours in the presence of nitrogen. This example employsa clad constitution in which Al is used for the conductive layer (B)making up the first conductive layer. Because the conductive layer (A)and the second conductive layer, both made of Ta, are formed to coverthe Al, the Ta layers function as blocking layers to prevent diffusionof Al atoms into other regions. The laser annealing converges the KrFexcimer laser beam of pulse oscillation type into a linear beam andapplies the beam to the impurity elements for activation. Performing thethermal annealing after the laser annealing produced a better result.This process also serves to anneal the regions where crystals weredestroyed by the ion doping and improve the crystallinity of theregions.

With the above processes completed, the gate electrode is formed whichcomprises the first conductive layer and the second conductive layer incontact with the first conductive layer, and the semiconductive layers204, 206 formed have the first impurity region that will make the LDDregion and the second impurity region that will make a source region anda drain region. The first impurity region has a region where it overlapsthe second conductive layer with the gate insulating film interposedtherebetween and a region where it does not. The p-channel TFT, on theother hand, is formed with a channel forming region, a source region anda drain region.

With the state of FIG. 3(B) obtained, the resist masks 258, 259 areremoved and a first interlayer insulating film 263 is formed to athickness of 1,000 nm. The first interlayer insulating film 263 may usea silicon oxide film, a silicon nitride film, a silicon oxynitride film,an organic resin film or a laminated layer of these. This example uses atwo-layer constitution, though not shown, in which a silicon nitridefilm is first formed to a thickness of 50 nm followed by a silicon oxidefilm 950 nm thick.

The first interlayer insulating film 263 is then patterned to formcontact holes in the source region and the drain region of each TFT.Then source lines 264, 265, 266 and drain lines 267, 268 are formed.Though not shown, this example forms these electrodes as wires of athree-layer constitution which comprises a Ti film 100 nm thick, an Alfilm, including Ti, 300 nm thick, and a Ti film 150 nm thick, all formedcontinuously by sputtering.

Then, a passivation film 269 is formed over the source lines 264, 265,266, the drain lines 267, 268 and the first interlayer insulating film263. The passivation film 269 is formed of a silicon nitride film 50 nmthick. Then, a second interlayer insulating film 270 of organic resin isformed to a thickness of about 1,000 nm. The organic resin film can usepolyimide, acrylics, polyimideamide, etc. The advantages of using anorganic resin film include the ease with which the deposition can bedone and a low relative dielectric constitution which in turn reducesparasitic capacitance, and an excellent flatness. Other organic resinfilms may be used. Here, polyimide of a type which thermally polymerizesafter application to the substrate is used and sintered at 300° C. toform the second interlayer insulating film.

In this way, the active matrix substrate having the CMOS circuit and thepixel area formed on the substrate 201, as shown in FIG. 3(C), ismanufactured. A storage capacitance is simultaneously formed on thedrain side of the pixel TFT.

EXAMPLE 2

This example shows an example case where, after the state shown in FIG.2(C) has been obtained by the same process as in the example 1, a partof the second conductive layer is removed by other method to form aregion where the first impurity region overlaps the second conductivelayer and a region where it does not.

First, as shown in FIG. 2(C), the resist masks 223, 224, 225, 226, 227used in the process of patterning the second conductive layer are usedas is to etch back a part of the second gate electrode by a length L5 asshown in FIG. 4(A).

This process was performed by dry etching. Depending on the material ofthe second conductive layer, a fluorine-based gas is basically used toachieve an isotropic etch and thereby remove those parts of the secondconductive layer beneath the resist masks. The etching gas, for example,is a CF₄ gas for Ta, a CF₄ or CCl₄ gas for Ti, and an SF₆ or NF₃ for Mo.

Then, as shown in FIG. 4(B), the second conductive layer is removed bythe length L5, i.e., 0.7 μm in this example. As a result, in then-channel TFT the first impurity region that will make the LDD regionhas a length of 1.5 μm (L6), in which a region in which the firstimpurity region overlaps the second conductive layer is formed at 0.8 μm(L4) and a region in which it does not is formed at 0.7 μm (L5).

The subsequent processes were performed in a manner similar to example 1to form an active matrix substrate shown in FIG. 4(C).

EXAMPLE 3

This example describes a process of manufacturing an active matrixliquid crystal display from an active matrix substrate formed in theexample 1 or example 2.

In the active matrix substrate as shown in FIG. 3(C) or FIG. 4(C),contact holes are formed in the second interlayer insulating film 270 toreach the drain line 268, and then a pixel electrode 271 is formed. Thepixel electrode 271 uses a transparent conductive film for atransmission type liquid crystal display and a metal film for areflection type liquid crystal display. Here, in order to fabricate atransmission type liquid crystal display, an indium-tin oxide (ITO) filmis sputtered to a thickness of 100 nm to form the pixel electrode 271.

After the state of FIG. 5(A) is obtained, an alignment film 272 isformed over the second interlayer insulating film 270 and the pixelelectrode 271. Generally, the alignment film for the liquid crystaldisplay element often uses a polyimide resin. A counter substrate 273 isformed with a transparent conductive film 274 and an alignment film 275.The alignment film, after being formed, is rubbed to orientate liquidcrystal molecules parallelly at a predetermined pretilt angle.

After constitution of the above process, the active matrix substrateformed with the pixel area and the CMOS circuits and the countersubstrate are sealed together by a known cell assembly process with asealing material and spacers (neither of them shown) interposedtherebetween. Then, a liquid crystal material 276 is injected betweenthe substrates and completely sealed with a sealant (not shown). Now,the active matrix liquid crystal display as shown in FIG. 5(B) iscompleted.

Next, the constitution of the active matrix liquid crystal display ofthis example will be explained by referring to FIGS. 7 and 8. FIG. 7 isa perspective view of the active matrix liquid crystal display of thisexample. The active matrix substrate comprises a pixel area 701, a scan(gate) line driving circuit 702 and a data (source) line driving circuit703. A pixel TFT 700 in the pixel area is an n-channel TFT and thedriving circuits provided around the pixel area are basically CMOScircuits. The scan (gate) line driving circuit 702 and the data (source)line driving circuit 703 are connected to the pixel area 701 by gatelines 802 and source lines 803, respectively.

FIG. 8(A) shows a top view of the pixel area 701 for almost one pixel.The pixel area is provided with an n-channel TFT. A gate electrode 820formed contiguous to a gate line 802 crosses an underlying semiconductorlayer 801 with a gate insulating film not shown interposed therebetween.Though not shown, the semiconductor layer is formed with a firstimpurity region and source and drain regions formed as a second impurityregion. On the drain side of the pixel TFT is formed a storagecapacitance 807 which comprises the semiconductor layer, the gateinsulating film, and an electrode made of the same material as the firstand second conductive layers. A capacitive line 821 connected to thestorage capacitance 807 is laid parallel to the gate line 802. Thecross-sectional constitution along the line A-A′ of FIG. 8(A)corresponds to the cross sections of the pixel area shown in FIGS. 3(C)and 4(C).

In the CMOS circuit shown in FIG. 8(B), gate electrodes 813 and 814extending from a gate line 819 cross underlying semiconductor layers 810and 812, respectively, with a gate insulating film not shown interposedtherebetween. Though not shown, the semiconductor layer of the n-channelTFT is similarly formed with a first impurity region and source anddrain regions formed as a second impurity region. The semiconductorlayer of the p-channel TFT is formed with source and drain regionsformed as a third impurity region. As to the positional relationship,the cross-sectional constitution along the line B-B′ corresponds to thecross sections of the pixel area shown in FIG. 3(C) or FIG. 4(C).

While this example has the pixel TFT 700 arranged in a double gateconstitution, it may be formed as a single gate constitution, or amultigate constitution such as a triple gate constitution. Theconstitution of the active matrix substrate is not limited to that ofthis example. The constitution of this invention is characterized by theconstitution of the gate electrode, by the constitution of the sourceand drain regions of the semiconductor layer formed under the gateelectrode with a gate insulating film interposed therebetween, and bythe constitution of other impurity regions. Other constitution can beappropriately determined by a person implementing this invention.

EXAMPLE 4

This example describes an example case which is similar in the processto the example 1, but in which the constitution of the second conductivelayer of the gate electrode in the n- and p-channel TFTs of the pixelTFT and CMOS circuit differs from that of the example 1. As shown inFIG. 6(A), the second conductive layers 280, 281, 282 and 283 are incontact with the first conductive layers and extend only on the drainside of each TFT. Even with the second conductive layers arranged inthis configuration, a high electric field generated on the drain sidecan be alleviated by forming first impurity regions 229, 236 and 240 incontact with the drain regions 231, 238 and 241 of the n-channel TFTs.

The process of this example is basically similar to that of the example1, except that only the photoresist mask used in the patterning processis changed to form the second conductive layer, with no other processeschanged. However, the first impurity region 229 of the n-channel TFT isformed only on the drain region side.

The first impurity region formed has a region where it overlaps thesecond conductive layer with the gate insulating film interposedtherebetween and a region where it does not. The p-channel TFT, on theother hand, is formed with a channel forming region 260, a source region261 and a drain region 262. Then, a first interlayer insulating film263, source lines 264, 265 and 266, drain lines 267 and 268, and apassivation film 269 are formed. A second interlayer insulating film 270of organic resin is then formed.

As a result, as shown in FIG. 6(B), the first impurity region formed inthe n-channel TFT that will make the LDD region has a region L4 long inwhich the first impurity region overlaps the second conductive layer anda region L5 long in which it does not. The subsequent processes areperformed in a way similar to the example 1 to form the active matrixsubstrate shown in FIG. 4(C). By using such an active matrix substrate,performing the method described in the example 3 can similarlymanufacture a liquid crystal display.

EXAMPLE 5

This example describes an example case in which a crystallinesemiconductor film used as the semiconductor layer in the examples 1, 2and 4 is formed by thermal annealing using a catalytic element. When acatalytic element is used, it is preferred that techniques disclosed inJP-A 7-130652 and 8-78329 be used.

Here, an example of applying the technique disclosed in JP-A 7-130652 tothis invention is shown in FIG. 9. First, a substrate 901 is depositedwith a silicon oxide film 902, over which an amorphous silicon film 903is formed. Further, a nickel acetate solution containing 10 ppm byweight of nickel is applied to the amorphous silicon film to form anickel-containing layer 904 (FIG. 9(A)).

Next, the substrate is heated at 500° C. for one hour fordehydrogenation. After this, the substrate is heat-treated in atemperature range of 500 to 650° C. for 4 to 12 hours, for example, at550° C. for 8 hours, to form a crystal silicon film 905. The crystalsilicon film 905 thus formed has a very good crystallinity (FIG. 9(B)).

The technique disclosed in JP-A 8-78329 enables selectivecrystallization of an amorphous semiconductor film by selectively addinga catalytic element. An example case of applying this technique to thepresent invention will be explained by referring to FIG. 10.

First, the glass substrate 1001 is deposited with a silicon oxide film1002, over which an amorphous silicon film 1003 and a silicon oxide film1004 are successively formed. At this time, the thickness of the siliconoxide film 1004 is set to 150 nm.

Then, the silicon oxide film 1004 is patterned to selectively form holes1005, after which a solution of nickel acetate containing 10 ppm byweight of nickel is applied to form a nickel-containing layer 1006 onthe surface. The nickel-containing layer 1006 contacts the amorphoussilicon film 1002 only at the bottom of the holes 1005 (FIG. 10(A)).

Next, the substrate is heat-treated in the temperature range of 500 to650° C. for 4 to 24 hours, for example at 570° C. for 14 hours, to forma crystal silicon film 1007. In this crystallization process, thoseportions of the amorphous silicon film which are contacted by nickelcrystallize first, with the crystallization propagating in thehorizontal direction. The crystal silicon film 1007 thus formed is anaggregation of bar- or needle-like crystals which, when viewedmacroscopically, grow in a particular orientation, thus offering theadvantage of aligned crystallinity (FIG. 10(B)).

The catalytic elements that can be used in the above two techniquesinclude iron (Fe), palladium (Pd), tin (Sn), lead (Pb), cobalt (Co),platinum (Pt), copper (Cu) and gold (Au) in place of nickel (Ni).

Using the above techniques to form a crystal semiconductor film(including a crystal silicon film and a crystal silicon-germanium film)and patterning the crystal semiconductor film can form a semiconductorlayer of a crystal TFT. The TFT manufactured from the crystalsemiconductor film by using the technique of this example has anexcellent characteristic but requires to have high reliability. However,by using the TFT constitution of this invention, it is possible tomanufacture a TFT taking full advantage of the technique of thisexample.

EXAMPLE 6

This example describes an example case in which the method of forming asemiconductor layer used in the examples 1, 2 and 4 involves using anamorphous semiconductor film as an initial film, transforming theamorphous semiconductor film into a crystal semiconductor film by usingthe catalytic element described above, and removing the catalyticelement from the crystal semiconductor film. This example uses thetechnique disclosed in JP-A 10-247735, 10-135468 or 10-135469.

The technique described in the above official gazettes is the one toremove the catalytic element used for crystallizing the amorphoussemiconductor film by using the gettering action of phosphorus after theamorphous semiconductor film is crystallized. The use of this techniquecan reduce the concentration of the catalytic element in the crystalsemiconductor film down to 1×10¹⁷ atoms/cm³ or less, preferably to1×10¹⁶ atoms/cm⁻¹.

The constitution of this example will be explained by referring to FIG.11. A glass substrate 1101 is an alkali-free glass substrate representedby 1737 substrate of Corning make. FIG. 11(A) shows a constitution inwhich a base film 1102 and a crystal silicon film 1103 are formed byusing the crystallization technique shown in the example 5. Then, asilicon oxide film 1104 as a mask is formed over the surface of thecrystal silicon film 1103 to a thickness of 150 nm and then patterned toform holes in which to expose the crystal silicon film. A phosphorusadding process is performed to form regions 1105 where the crystalsilicon film is doped with phosphorus.

In this state, the substrate is heated in the presence of nitrogen inthe temperature range of 550 to 800° C. for 5 to 24 hours, for exampleat 600° C. for 12 hours. The regions 1105 in which the crystal siliconfilm is doped with phosphorus act as gettering sites to cause thecatalytic element remaining in the crystal silicon film 1103 to besegregated in the regions 1105 doped with phosphorus.

Then, the silicon oxide film 1104 for masking and the phosphorus-dopedregions 1105 are etched away to obtain a crystal silicon film whoseconcentration of the catalytic element used in the crystallizationprocess is lowered to 1×10¹⁷ atoms/cm³. This crystal silicon film can bedirectly used as the semiconductor film of TFT of this invention shownin the examples 1, 2 and 4.

EXAMPLE 7

This example describes another example case in which a semiconductorlayer and a gate insulating film are formed in the process ofmanufacturing a TFT of this invention shown in the examples 1, 2 and 4.The constitution of this example will be explained by referring to FIG.12.

Here, a substrate with heat resistance of at least about 700 to 1,100°C. is needed and a quartz substrate 1201 is used. By using the techniqueshown in the example 5, a crystal semiconductor film is formed. Totransform this crystal semiconductor film into the semiconductor layerof the TFT, the crystal semiconductor film is patterned into islands toform semiconductor layers 1202, 1203, which are then covered with a gateinsulating film 1204 composed mainly of silicon oxide. In this example,a silicon oxynitride film is formed by a plasma CVD method to athickness of 70 nm (FIG. 12(A)).

Then, the substrate is heat-treated in an atmosphere containing halogen(typically chlorine) and oxygen. This is done at 950° C. for 30 minutesin this example. The heat treatment temperature can be selected in therange of 700 to 1,100° C. and the processing time may range from 10minutes to 8 hours (FIG. 12(B)).

As a result, under the condition of this example, an interface betweenthe semiconductor layers 1202, 1203 and the gate insulating film 1204 isthermally oxidized to form a gate insulating film 1207. During theprocess of oxidation in the presence of halogen gas, impuritiescontained in the gate insulating film 1204 and the semiconductor layers1202, 1203, particularly metal impurity elements, react with halogen toform compounds, thus releasing the impurities in a gas phase.

The gate insulating film 1207 fabricated in the process described abovehas high insulation resistance and the interface between thesemiconductor layers 1205, 1206 and the gate insulating film 1207 isvery good. To realize the constitution of the TFT according to thisinvention, the subsequent processes need only to follow those of theexamples 1, 2 and 4.

EXAMPLE 8

This example shows in FIG. 13 an example of manufacturing a crystal TFTby a sequence of processes different from that of the examples 1. Thesemiconductor layers 204, 205, 206 of the example 1 shown in FIG. 2(A)use crystal silicon films fabricated by the method shown in the example5. The catalytic element used in the crystallization process remains ina small amount in the semiconductor layers. The subsequent processes upto the step in FIG. 3(B) of doping a p-type impurity are performedaccording to the example 1. Then the resist masks 258, 259 are removed.

At this time, as shown in FIG. 13, the source regions 230, 237 and drainregions 231, 238, 241 of the n-channel TFT and the source region 261 anddrain region 262 of the p-channel TFT are both doped with phosphorusduring the process of FIG. 2(C). According to the example 1, thephosphorus concentration is 1×10¹⁹-1×10²¹ atoms/cm³.

In this state, the substrate is heat-treated in the presence of nitrogenat 500 to 800° C. for 1 to 24 hours, for example, at 600° C. for 12hours. This process can activate the n- and p-type impurity elementsadded. Further, the phosphorus-doped regions become gettering sites tosegregate the catalytic elements remaining after the crystallizationprocess. As a result, the catalytic elements can be removed from thechannel forming regions.

After completion of the process of FIG. 13, the subsequent steps areperformed according to the example 1 to form the state shown in FIG.3(C), thus manufacturing the active matrix substrate. By using such anactive matrix substrate, performing the process as described in theexample 3 can produce the similar liquid crystal display.

EXAMPLE 9

This example shows in FIG. 14 example constitution of the gate electrodein the TFT according to this invention. The gate electrode comprises afirst conductive layer and a second conductive layer formed in contactwith the first conductive layer. The first conductive layer comprisesone or more conductive layers.

FIG. 14(A) shows a constitution in which a conductive layer (A) of thefirst conductive layer formed in contact with a gate insulating film isformed of a Ta film, a conductive layer (B) deposited over theconductive layer (A) is formed of Ti, a conductive layer (C) is formedof a film made mainly of Al, and a fourth conductive layer on top ismade of Ti. It is preferred that the conductive layer (A) have athickness of 30 to 200 nm and other conductive layers a thickness of 50to 100 nm.

The conductive layer (A) in contact with the gate insulating film servesas a barrier layer to prevent chemical elements making up the overlyingconductive layer from diffusing into the gate insulating film and shouldpreferably use a high melting point metal such as Ti, Ta, W and Ho ortheir alloy material. The conductive layer (C) formed in FIG. 14(A) ismade of a film mainly containing Al and used to reduce the resistivityof the gate electrode. To increase flatness of the Al film formed, it isdesirable to use an Al alloy film containing 0.1-5 atm % of elementssuch as scandium (Sc), Ti and silicon (Si). In either case, when thisinvention is applied to a liquid crystal display of 10-inch or largersize, it is preferred that a material with a low resistivity containingmainly Al or Cu be used to reduce the resistance of the gate electrode.Further, the first conductive layer and the second conductive layerformed in contact with the gate insulating film preferably use a highmelting point metal such as Ti, Ta, W and Mo or their alloy material.

FIG. 14(B) shows another example of the gate electrode constitution inwhich the first conductive layer is formed of a single layer of Mo-Walloy film and the second conductive layer is formed of a Ti-Mo alloyfilm. The first conductive layer may be formed in a single layer whosethickness may be set in the range of 50 to 100 nm.

FIG. 14(C) shows a constitution in which the conductive layer (A) makingup the first conductive layer is formed of Ti film, the conductive layer(B) is formed of a film having copper (Cu) as a major component, and theconductive layer (C) is formed of Ti film. The resistivity of the gateelectrode can also be lowered by using Cu film instead of Al film. Thesecond conductive layer is formed of Ti film.

FIG. 14(D) shows a constitution in which the conductive layer (A) makingup the first conductive layer is formed of Ti film, the conductive layer(B) is formed of a film having Al as a major component, and theconductive layer (C) is formed of Ti film. The second conductive layeris formed of Mo film.

EXAMPLE 10

This example explains, by referring to FIG. 18, an example case wherethe TFT forming area and the line area have different lengthscorresponding to L4 of FIG. 16.

In FIG. 18, over the semiconductor layer 140 are formed a firstconductive layer 141 and a second conductive layer 142. The secondconductive layer 142 is formed to cover and hide the first conductivelayer 141. In this specification the length of a portion that does notoverlap the first conductive layer 141 is defined as L4.

In this example, the length of L4 in the TFT forming area (on thesemiconductor layer) (denoted W_(LDD)) is set to 0.1 to 2 μm (typically0.3 to 1.5 μm). The length L4′ in the line area (on other than thesemiconductor layer) (denoted W_(L)) is set to 0.05 to 0.5 μm (typically0.1 to 0.3 μm).

This example is characterized in that the line width of the secondconductive layer is narrower in the line forming area than in the TFTforming area. In the line forming area a region corresponding to L4 isnot necessary but hinders an increase in the packing density of lines,which means that the line width should be made as narrow as possible.

Hence, the use of the constitution of this example facilitatesincreasing the integration of lines, contributing to a higherintegration of semiconductor device. The constitution of this examplecan be combined freely with any constitution of the examples 1 to 12.

EXAMPLE 11

This example explains, by referring to FIG. 21, an example case in whichthis invention is applied to a display (organic EL display) using anactive matrix type organic electroluminescence (organic EL) material.FIG. 21(A) shows a circuit diagram of an active matrix type organic ELdisplay having a display area formed on a glass substrate and drivingcircuits formed along the periphery of the display area. The organic ELdisplay comprises a display area 11 formed on the substrate, anX-direction peripheral driving circuit 12, and a Y-direction peripheraldriving circuit 13. The display area 11 comprises a switching TFT 30, astorage capacitance 32, a current control TFT 31, an organic EL element33, X-direction signal lines 18 a, 18 b, power lines 19 a, 19 b, andY-direction signal lines 20 a, 20 b, 20 c.

FIG. 21(B) shows a top view of almost one pixel. The switching TFT 30and the current control TFT 31 are formed in the same way as in then-channel TFT shown in FIG. 3(C) of example 1.

FIG. 22 is a cross section taken along the line B-B′ of FIG. 21(B),showing the cross section of the switching TFT 30, storage capacitance32, current control TFT 31 and organic EL element portion. Over asubstrate 40, base films 41, 42, gate insulating film 45, firstinterlayer insulating film 46, gate electrodes 47, 48, capacitance line49, source and drain lines 18 a, 19 a, 51, 52, and second interlayerinsulating film 50 are formed in the same way as in the example 1. Then,over these layers is formed a third interlayer insulating film 53 in away similar to the second interlayer insulating film 50. A contact holereaching the drain line 52 is formed, after which a pixel electrode 54made of a transparent conductive film is formed. The organic EL elementportion comprises the pixel electrode 54; an organic EL layer 55overlying the pixel electrode and the third interlayer insulating film53; and a first electrode 56 made of Mg—Ag compound and a secondelectrode 57 made of Al, formed over the organic EL layer 55. If a colorfilter, though not shown, is used, a color display is possible. Byapplying the active matrix substrate manufacturing method shown in theexamples 1 to 10, the active matrix type organic EL display can befabricated easily.

TFT of the active matrix type organic EL display shown in this examplecan be manufactured according to the method of the example 1. The TFTconstitution of this example can suitably be applied to an organic ELdisplay described above.

EXAMPLE 12

This example demonstrates another process for producing an EL(electroluminescence) display device according to the invention of thepresent application.

FIG. 23A is a top view showing an EL display device, which was producedaccording to the invention of the present application. In FIG. 23A,there are shown a substrate 4010, a pixel part 4011, a driving circuitfrom the source 4012, and a driving circuit from the gate 4013, eachdriving circuit connecting to wirings 4014-4016 which reach FPC 4017leading to external equipment.

The pixel part, preferably together with the driving circuit, isenclosed by a covering material 6000, a sealing material (or housingmaterial) 7000, and an end-sealing material (or second sealing material)7001.

FIG. 23B is a sectional view showing the structure of the EL displaydevice in this Example. There is shown a substrate 4010, an underlyingcoating 4021, a TFT 4022 for the driving circuit, and a TFT 4023 for thepixel unit. (The TFT 4022 shown is a CMOS circuit consisting of ann-channel type TFT and a p-channel type TFT. The TFT 4023 shown is theone, which controls current to the EL element.) These TFTs may be of anyknown structure (top gate structure or bottom gate structure).

Incidentally, the present invention is used in the TFT 4022 for thedriving circuit and the TFT 4023 for the pixel unit.

Upon completion of TFT 4022 (for the driving circuit) and TFT 4023 (forthe pixel unit), with their active layer being the semiconductor layerformed according to the invention of the present application, a pixelelectrode 4027 is formed on the interlayer insulating film (planarizingfilm) 4026 made of a resin. This pixel electrode is a transparentconductive film, which is electrically connected to the drain of TFT4023 for the pixel unit. The transparent conductive film may be formedfrom a compound (called ITO) of indium oxide and tin oxide or a compoundof indium oxide and zinc oxide. On the pixel electrode 4027 is formed aninsulating film 4028, in which is formed an opening above the pixelelectrode 4027.

Subsequently, the EL layer 4029 is formed. It may be of single-layerstructure or multi-layer structure by freely combining known ELmaterials such as injection layer, hole transport layer, light emittinglayer, electron transport layer, and electron injection layer. Any knowntechnology may be available for such structure. The EL material iseither a low-molecular material or a high-molecular material (polymer).The former may be applied by vapor deposition, and the latter may beapplied by a simple method such as spin coating, printing, or ink-jetmethod.

In this example, the EL layer is formed by vapor deposition through ashadow mask. The resulting EL layer permits each pixel to emit lightdiffering in wavelength (red, green, and blue). This realizes the colordisplay. Alternative systems available include the combination of colorconversion layer (CCM) and color filter and the combination of whitelight emitting layer and color filter. Needless to say, the EL displaydevice may be monochromatic.

On the EL layer is formed a cathode 4030. Prior to this step, it isdesirable to clear moisture and oxygen as much as possible from theinterface between the EL layer 4029 and the cathode 4030. This objectmay be achieved by forming the EL layer 4029 and the cathode 4030consecutively in a vacuum, or by forming the EL layer 4029 in an inertatmosphere and then forming the cathode 4030 in the same atmospherewithout admitting air into it. In this Example, the desired film wasformed by using a film-forming apparatus of multi-chamber system(cluster tool system).

The multi-layer structure composed of lithium fluoride film and aluminumfilm is used in this Example as the cathode 4030. To be concrete, the ELlayer 4029 is coated by vapor deposition with a lithium fluoride film (1nm thick) and an aluminum film (300 nm thick) sequentially. Needless tosay, the cathode 4030 may be formed from MgAg electrode which is a knowncathode material. Subsequently, the cathode 4030 is connected to awiring 4016 in the region indicated by 4031. The wiring 4016 to supply aprescribed voltage to the cathode 4030 is connected to the FPC 4017through an electrically conductive paste material 4032.

The electrical connection between the cathode 4030 and the wiring 4016in the region 4031 needs contact holes in the interlayer insulating film4026 and the insulating film 4028. These contact holes may be formedwhen the interlayer insulating film 4026 undergoes etching to form thecontact hole for the pixel electrode or when the insulating film 4028undergoes etching to form the opening before the EL layer is formed.When the insulating film 4028 undergoes etching, the interlayerinsulating film 4026 may be etched simultaneously. Contact holes of goodshape may be formed if the interlayer insulating film 4026 and theinsulating film 4028 are made of the same material.

Then, a passivation film 6003, a filling material 6004 and a coveringmaterial 6000 are formed so that these layers cover the EL element.

Furthermore, the sealing material 7000 is formed inside of the coveringmaterial 6000 and the substrate 4010 such as surrounding the EL element,and the end-sealing material 7001 is formed outside of the sealingmaterial 7000.

The filling material 6004 is formed to cover the EL element and alsofunctions as an adhesive to adhere to the covering material 6000. As thefilling material 6004, PVC (polyvinyl chloride), an epoxy resin, asilicon resin, PVB (polyvinyl butyral), or EVA (ethylenvinyl acetate)can be utilized. It is preferable to form a desiccant in the fillingmaterial 6004, since a moisture absorption can be maintained.

Also, spacers can be contained in the filling material 6004. It ispreferable to use sperical spacers comprising barium oxide to maintainthe moisture absorption in the spacers.

In the case of that the spaces are contained in the filling material,the passivasion film 6003 can relieve the pressure of the spacers. Ofcourse, the other film different from the passivation film, such as anorganic resin, can be used for relieving the pressure of the spacers.

As the covering material 6000, a glass plate, an aluminum plate, astainless plate, a FRP (Fiberglass-Reinforced Plastics) plate, a PVF(polyvinyl fluoride) film, a Mylar film, a polyester film or an acrylfilm can be used. In a case that PVB or EVA is employed as the fillingmaterial 6004, it is preferable to use an aluminum foil with a thicknessof some tens of μm sandwiched by a PVF film or a Mylar film.

It is noted that the covering material 6000 should have a lighttransparency with accordance to a light emitting direction (a lightradiation direction) from the EL element.

The wiring 4016 is electrically connected to FPC 4017 through the gapbetween the sealing material 7000 and the end-sealing material 7001, andthe substrate 4010. As in the wiring 4016 explained above, other wirings4014 and 4015 are also electrically connected to FPC 4017 under thesealing material 4018.

EXAMPLE 13

In this embodiment, another EL display device having a differentstructure from the embodiment 12 is explained, as shown in FIGS. 24A and24B. The same reference numerals in FIGS. 24A and 24B as in FIGS. 23Aand 23B indicate same constitutive elements, so an explanation isomitted.

FIG. 24A shows a top view of the EL module in this embodiment and FIG.24B shows a sectional view of A-A′ of FIG. 24A.

According to Embodiment 12, the passivation film 6003 is formed to covera surface of the EL element.

The filling material 6004 is formed to cover the EL element and alsofunctions as an adhesive to adhere to the covering material 6000. As thefilling material 6004, PVC (polyvinyl chloride), an epoxy resin, asilicon resin, PVB (polyvinyl butyral), or EVA (ethylenvinyl acetate)can be utilized. It is preferable to form a desiccant in the fillingmaterial 6004, since a moisture absorption can be maintained.

Also, spacers can be contained in the filling material 6004. It ispreferable to use sperical spacers comprising barium oxide to maintainthe moisture absorption in the spacers.

In the case of that the spaces are contained in the filling material,the passivasion film 6003 can relieve the pressure of the spacers. Ofcourse, the other film different from the passivation film, such as anorganic resin, can be used for relieving the pressure of the spacers.

As the covering material 6000, a glass plate, an aluminum plate, astainless plate, a FRP (Fiberglass-Reinforced Plastics) plate, a PVF(polyvinyl fluoride) film, a Mylar film, a polyester film or an acrylfilm can be used. In a case that PVB or EVA is employed as the fillingmaterial 6004, it is preferable to use an aluminum foil with a thicknessof some tens of μm sandwiched by a PVF film or a Mylar film.

It is noted that the covering material 6000 should have a lighttransparency with accordance to a light emitting direction (a lightradiation direction) from the EL element.

Next, the covering material 6000 is adhered using the filling material3404. Then, the flame material 6001 is attached to cover side portions(exposed faces) of the filling material 6004. The flame material 6001 isadhered by the material 6002, a light curable resin is preferable. Also,a thermal curable resin can be employed if a heat resistance of the ELlayer is admitted. It is preferable for the sealing material 6002 not topass moisture and oxygen. In addition, it is possible to add a desiccantinside the sealing material 6002.

The wiring 4016 is electrically connected to FPC 4017 through the gapbetween the sealing material 6002 and the substrate 4010. As in thewiring 4016 explained above, other wirings 4014 and 4015 are alsoelectrically connected to FPC 4017 under the sealing material 6002.

EXAMPLE 14

In this example, the structure of the pixel region in the panel isillustrated in more detail. FIG. 25 shows the cross section of the pixelregion; FIG. 26A shows the top view thereof; and FIG. 26B shows thecircuit pattern for the pixel region. In FIG. 25, FIG. 26A and FIG. 26B,the same reference numerals are referred to for the same parts, as beingcommon thereto.

In FIG. 25, the switching TFT 3502 formed on the substrate 3501 is NTFTof the invention (cf. Examples 1 to 11). In this Example, it has adouble-gate structure, but its structure and fabrication process do notso much differ from the structures and the fabrication processesillustrated hereinabove, and their description is omitted herein.However, the double-gate structure of the switching TFT 3502 hassubstantially two TFTs as connected in series, and therefore has theadvantage of reducing the off-current to pass therethrough. In thisExample, the switching TFT 3502 has such a double-gate structure, but isnot limitative. It may have a single-gate structure or a triple-gatestructure, or even any other multi-gate structure having more than threegates. As the case may be, the switching TFT 3502 may be PTFT of theinvention.

The current-control TFT 3503 is NTFT of the invention. The drain wire3535 in the switching TFT 3502 is electrically connected with the gateelectrode 3537 in the current-control TFT, via the wire 3536therebetween. The wire indicated by 3538 is a gate wire for electricallyconnecting the gate electrodes 3539 a and 3539 b in the switching TFT3502.

It is very important that the current-control TFT 3503 has the structuredefined in the invention. The current-control TFT is a unit forcontrolling the quantity of current that passes through the EL device.Therefore, a large quantity of current passes through it, and the unit,current-control TFT has a high risk of thermal degradation anddegradation with hot carriers. To this unit, therefore, the structure ofthe invention is extremely favorable, in which an LDD region is soconstructed that the gate electrode overlaps with the drain area in thecurrent-control TFT, via a gate-insulating film therebetween.

In this Example, the current-control TFT 3503 is illustrated to have asingle-gate structure, but it may have a multi-gate structure withplural TFTs connected in series. In addition, plural TFTs may beconnected in parallel so that the channel-forming region issubstantially divided into plural sections. In the structure of thattype, heat radiation can be effected efficiently. The structure isadvantageous for protecting the device with it from thermaldeterioration.

As in FIG. 26A, the wire to be the gate electrode 3537 in thecurrent-control TFT 3503 overlaps with the drain wire 3540 therein inthe region indicated by 3504, via an insulating film therebetween. Inthis state, the region indicated by 3504 forms a capacitor. Thecapacitor 3504 functions to retain the voltage applied to the gate inthe current-control TFT 3503. The drain wire 3540 is connected with thecurrent supply line (power line) 3501, from which a constant voltage isall the time applied to the drain wire 3540.

On the switching TFT 3502 and the current-control TFT 3503, formed is afirst passivation film 3541. On the film 3541, formed is a planarizingfilm 3542 of an insulating resin. It is extremely important that thedifference in level of the layered parts in TFT is removed throughplanarization with the planarizing film 3542. This is because the ELlayer to be formed on the previously formed layers in the later step isextremely thin, and if there exist a difference in level of thepreviously formed layers, the EL device will be often troubled by lightemission failure. Accordingly, it is desirable to previously planarizeas much as possible the previously formed layers before the formation ofthe pixel electrode thereon so that the EL layer could be formed on theplanarized surface.

The reference numeral 3543 indicates a pixel electrode (a cathode in theEL device) of an electroconductive film with high reflectivity. Thepixel electrode 3543 is electrically connected with the drain in thecurrent-control TFT 3503. It is preferable that the pixel electrode 3543is of a low-resistance electroconductive film of an aluminium alloy, acopper alloy or a silver alloy, or of a laminate of those films.Needless-to-say, the pixel electrode 3543 may have a laminate structurewith any other electroconductive films.

In the recess (this corresponds to the pixel) formed between the banks3544 a and 3.544 b of an insulating film (preferably of a resin), thelight-emitting layer 44 is formed. In the illustrated structure, onlyone pixel is shown, but plural light-emitting layers could be separatelyformed in different pixels, corresponding to different colors of R(red), G (green) and B (blue). The organic EL material for thelight-emitting layer may be any π-conjugated polymer material. Typicalpolymer materials usable herein include polyparaphenylenevinylene (PVV)materials, polyvinylcarbazole (PVK) materials, polyfluorene materials,etc.

Various types of PVV-type organic EL materials are known, such as thosedisclosed in “H. Shenk, H. Becker, O. Gelsen, E. Klunge, W. Kreuder, andH. Spreitzer; Polymers for Light Emitting Diodes, Euro DisplayProceedings, 1999, pp. 33-37” and in Japanese Patent Laid-Open No.92576/1998. Any of such known materials are usable herein.

Concretely, cyanopolyphenylenevinylenes may be used for red-emittinglayers; polyphenylenevinylenes may be for green-emitting layers; andpolyphenylenevinylenes or polyalkylphenylenes may be for blue-emittinglayers. The thickness of the film for the light-emitting layers may fallbetween 30 and 150 nm (preferably between 40 and 100 nm).

These compounds mentioned above are referred to merely for examples oforganic EL materials employable herein and are not limitative at all.The light-emitting layer may be combined with a charge transportationlayer or a charge injection layer in any desired manner to form theintended EL layer (this is for light emission and for carrier transferfor light emission).

Specifically, this Example is to demonstrate the example of usingpolymer materials to form light-emitting layers, which, however, is notlimitative. Apart from this, low-molecular organic EL materials may alsobe used for light-emitting layers. For charge transportation layers andcharge injection layers, further employable are inorganic materials suchas silicon carbide, etc. Various organic EL materials and inorganicmaterials for those layers are known, any of which are usable herein.

In this Example, a hole injection layer 46 of PEDOT (polythiophene) orPAni (polyaniline) is formed on the light-emitting layer 3545 to give alaminate structure for the EL layer. On the hole injection layer 46,formed is an anode 3547 of a transparent electroconductive film. In thisExample, the light having been emitted by the light-emitting layer 3545radiates therefrom in the direction toward the top surface (that is, inthe upward direction of TFT). Therefore, in this, the anode musttransmit light. For the transparent electroconductive film for theanode, usable are compounds of indium oxide and tin oxide, and compoundsof indium oxide and zinc oxide. However, since the anode is formed afterthe light-emitting layer and the hole injection layer having poor heatresistance have been formed, it is preferable that the transparentelectroconductive film for the anode is of a material capable of beingformed into a film at as low as possible temperatures.

When the anode 3547 is formed, the EL device 3505 is finished. The ELdevice 3505 thus fabricated herein indicates a capacitor comprising thepixel electrode (cathode) 3543, the light-emitting layer 3545, the holeinjection layer 4 and the anode 3547. As in FIG. 36A, the region of thepixel electrode 43 is nearly the same as the area of the pixel.Therefore, in this, the entire pixel functions as the EL device.Accordingly, the light utility efficiency of the EL device fabricatedherein is high, and the device can display bright images.

In this Example, a second passivation film 3548 is formed on the anode3547. For the second passivation film 3548, preferably used is a siliconnitride film or a silicon oxynitride film. The object of the film 3548is to insulate the EL device from the outward environment. The film 48has the function of preventing the organic EL material from beingdegraded through oxidation and has the function of preventing it fromdegassing. With the second passivation film 3548 of that type, thereliability of the EL display device is improved.

As described hereinabove, the EL display panel of the inventionfabricated in this Example has a pixel region for the pixel having theconstitution as in FIG. 25, and has the switching TFT through which theoff-current to pass is very small to a satisfactory degree, and thecurrent-control TFT resistant to hot carrier injection. Accordingly, theEL display panel fabricated herein has high reliability and can displaygood images.

The constitution of this Example can be combined with any constitutionof Examples 1 to 11 in any desired manner. Incorporating the EL displaypanel of this Example into the electronic appliance of Example 19 as itsdisplay part is advantageous.

EXAMPLE 15

This Example is to demonstrate a modification of the EL display panel ofExample 14, in which the EL device 3505 in the pixel region has areversed structure. For this Example, referred to is FIG. 27. Theconstitution of the EL display panel of this Example differs from thatillustrated in FIG. 26A only in the EL device part and thecurrent-control TFT part. Therefore, the description of the other partsexcept those different parts is omitted herein.

In FIG. 27, the current-control TFT 3701 may be PTFT of the invention.For the process of forming it, referred to is that of Example 1 though11.

In this Example, the pixel electrode (anode) 3550 is of a transparentelectroconductive film. Concretely, used is an electroconductive film ofa compound of indium oxide and zinc oxide. Needless-to-say, also usableis an electroconductive film of a compound of indium oxide and tinoxide.

After the banks 51 a and 51 b of an insulating film have been formed, alight-emitting layer 3552 of polyvinylcarbazole is formed between themin a solution coating method. On the light-emitting layer 3552, formedare an electron injection layer 3553 of acetylacetonatopotassium(hereinafter acacK), and a cathode 3554 of an aluminium alloy. In thiscase, the cathode 3554 serves also as a passivation film. Thus isfabricated the EL device 3701.

In this Example, the light having been emitted by the light-emittinglayer radiates in the direction toward the substrate with TFT formedthereon, as in the direction of the arrow illustrated.

The constitution of this Example can be combined with any constitutionof Examples 1 through 11 in any desired manner. Incorporating the ELdisplay panel of this Example into the electronic appliance of Example19 as its display part is advantageous.

EXAMPLE 16

This Example is to demonstrate modifications of the pixel with thecircuit pattern of FIG. 26B. The modifications are as in FIG. 28A toFIG. 28C. In this Example illustrated in those FIG. 28A through FIG.28C, 3801 indicates the source wire for the switching TFT 3802; 3803indicates the gate wire for the switching TFT 3802; 3804 indicates acurrent-control TFT; 3805 indicates a capacitor; 3806 and 3808 indicatecurrent supply lines; and 3807 indicates an EL device.

In the example of FIG. 28A, the current supply line 3806 is common tothe two pixels. Specifically, this example is characterized in that twopixels are lineal-symmetrically formed with the current supply line 3806being the center between them. Since the number of current supply linescan be reduced therein, this example is advantageous in that the pixelpattern can be much finer and thinner.

In the example of FIG. 28B, the current supply line 3808 is formed inparallel to the gate wire 3803. Specifically, in this, the currentsupply line 3808 is so constructed that it does not overlap with thegate wire 3803, but is not limitative. Being different from theillustrated case, the two may overlap with each other via an insulatingfilm therebetween so far as they are of different layers. Since thecurrent supply line 3808 and the gate wire 3803 may enjoy the commonexclusive area therein, this example is advantageous in that the pixelpattern can be much finer and thinner.

The structure of the example of FIG. 28C is characterized in that thecurrent supply line 3808 is formed in parallel to the gate wires 3803,like in FIG. 38B, and that two pixels are lineal-symmetrically formedwith the current supply line 3808 being the center between them. Inthis, it is also effective to provide the current supply line 3808 insuch a manner that it overlaps with any one of the gate wires 3803.Since the number of current supply lines can be reduced therein, thisexample is advantageous in that the pixel pattern can be much finer andthinner.

The constitution of this Example can be combined with any constitutionof Example 1 through 13 in any desired manner. Incorporating the ELdisplay panel having the pixel structure of this Example into theelectronic appliance of Example 19 as its display part is advantageous.

EXAMPLE 17

The figure of Example 14, that are illustrated in FIG. 26A and FIG. 26Bis provided with the capacitor 3504 which acts to retain the voltageapplied to the gate in the current-control TFT 3503. In the example,however, the capacitor 3504 may be omitted.

In the Example 14, the current-control TFT 3503 is NTFT of theinvention, as shown in Examples 1 through 11. Therefore, in the example,the LDD region is so formed that it overlaps the gate electrode with thegate-insulating film interposed therebetween. In the overlapped region,a parasitic capacitance is formed, as generally referred to as a gatecapacitance. The present example is characterized in that the parasiticcapacitance is positively utilized in place of the capacitor 3504.

The parasitic capacitance in question varies, depending on the area inwhich the gate electrode overlaps with the LDD region, and is thereforedetermined according to the length of the LDD region in the overlappedarea.

Also as illustrated in FIG. 28A, FIG. 28B and FIG. 28C of Example 16,the capacitor 3805 can be omitted.

The constitution of this Example can be combined with any constitutionof Examples 1 through 13 in any desired manner. Incorporating the ELdisplay panel having the pixel structure of the present example into theelectronic appliance of Example 19 as its display part is advantageous.

Carrying out the present invention, stable operation may be obtainedeven when the pixel TFT of the pixel matrix circuit is driven byapplying a gate voltage of 15 to 20 V. As a result, a semiconductordevice including a CMOS circuit made from a crystalline TFT,specifically, a pixel matrix circuit or a driver circuit disposed at itsperiphery in a liquid crystal display device or an EL display device maybe enhanced in reliability, providing a durable liquid crystal displaydevice or a durable EL display device against the long time use.

EXAMPLE 18

The liquid crystal display of this invention can use a variety of liquidcrystals other than a nematic liquid crystal. Examples of such liquidcrystals include those disclosed in papers entitled “Characteristics andDriving Scheme of Polymer-Stabilized Monostable FLCD Exhibiting FastResponse Time and High Contrast Ratio with Gray-Scale Capability” by H.Furue, et al. 1998, SID, “A Full-Color Thresholdless AntiferroelectricLCD Exhibiting Wide Viewing Angle with Fast Response Time” by T.Yoshida, et al. 1997, SID DIGEST, 841, and “Thresholdlessantiferroelectricity in liquid crystals and its application to displays”by S. Inui, et al. 1996, J. Mater. Chem. 6(4), 671-673, and thosedisclosed in U.S. Pat. No. 5,594,569.

FIG. 29 shows an electrooptical characteristic of a monostableferroelectric liquid crystal (FLC) exhibiting an isotropicphase/cholesteric phase/chiral smectic phase transition sequence whenthe FLC with the cone edge almost aligned to the rubbing direction isapplied with a DC voltage to cause the cholesteric phase/chiral smecticphase transition.

The display mode of the ferroelectric liquid crystal as shown in FIG. 29is called a “half-V switching mode.” In the graph of FIG. 29, theordinate represents a transmittivity (in arbitrary unit) and theabscissa an applied voltage. The “half-V switching mode” is detailed inpapers such as “Half-V Switching Mode FLCD,” by Terada, et al.proceedings of lectures in 46th Applied Physics Joint Conference, March1999, p 1316 and “Time-Division Full-Color LCD Using FerroelectricLiquid Crystal” by Yoshihara, Liquid Crystal, 3rd Vol., No. 3, p 190.

As shown in FIG. 29, it is seen that the use of such a ferroelectricliquid crystal enables low-voltage driving and gray-scale display. Theliquid crystal display of this invention can use a ferroelectric liquidcrystal exhibiting such an electrooptical characteristic.

A liquid crystal exhibiting an antiferroelectric phase in a particulartemperature range is called an antiferroelectric liquid crystal (AFLC).Among mixed liquid crystals containing an antiferroelectric liquidcrystal, there are thresholdless antiferroelectric mixed liquid crystalsthat exhibit an electrooptical response characteristic in which thetransmittivity changes continuously with respect to an electric field.Some thresholdless antiferroelectric mixed liquid crystals exhibit aso-called V-type electrooptical response characteristic in which thedrive voltage is about ±2.5 V (cell thickness about 1 to 2 μm).

The thresholdless antiferroelectric mixed liquid crystals generally havea high level of spontaneous polarization and therefore a high dielectricconstitution. Hence, when the thresholdless antiferroelectric mixedliquid crystals are used in the liquid crystal display, the pixels arerequired to have a relatively large storage capacitance. It is thereforepreferred that a thresholdless antiferroelectric mixed liquid crystalwith small spontaneous polarization be used.

By using such a thresholdless antiferroelectric mixed liquid crystal inthe liquid crystal display of this invention, a low voltage driving andtherefore a low power consumption can be realized.

EXAMPLE 19

This example describes semiconductor apparatus incorporating the activematrix liquid crystal display using TFT circuits of this invention.

Such semiconductor apparatus include portable information terminals(personal digital assistants, mobile computers, cellular phones, etc.),video cameras, still cameras, personal computers and television sets.Their examples are shown in FIGS. 15, 19 and 20.

FIG. 15(A) shows a cellular phone, which includes a body 2001, a voiceoutput section 2002, a voice input section 2003, a display 2004, anoperation switch 2005, and an antenna 2006. This invention can beapplied to the display 2004 having the voice output section 2002, thevoice input section 2003 and an active matrix substrate.

FIG. 15(B) shows a video camera, which includes a body 2101, a display2102, a voice input section 2103, an operation switch 2104, a battery2105, and a picture receiving section 2106. This invention can beapplied to the display 2102 and the picture receiving section 2106having the voice input section 2103, an active matrix substrate.

FIG. 15(C) shows a mobile computer, which includes a body 2201, a camerasection 2202, a picture receiving section 2203, an operation switch2204, and a display 2205. This invention can be applied to the picturereceiving section 2203 and the display 2205 having an active matrixsubstrate.

FIG. 15(D) shows a head-mounted display, which includes a body 2301, adisplay 2302, and an arm section 2303. This invention can be applied tothe display 2302 and, though not shown, to a signal control circuit.

FIG. 15(E) shows a rear type projector, which includes a body 2401, alight source 2402, a display 2403, a polarizing beam splitter 2404,reflectors 2405, 2406, and a screen 2407. This invention can be appliedto the display 2403.

FIG. 15(F) shows a portable book, which includes a body 2501, displays2502, 2503, a storage medium 2504, an operation switch 2505 and anantenna 2506, and designed to display data stored in minidisk (MD) andDVD and data received via antenna. The displays 2502, 2503 aredirect-view type displays that can apply this invention.

FIG. 19(A) shows a personal computer, which includes a body 2601 havingmicroprocessor and memory, an image input section 2602, a display 2603and a keyboard 2604. This invention can form the display 2603 and asignal processing circuit.

FIG. 19(B) shows a player that uses a recording medium storing a program(herein after referred to as a recording medium) and which includes abody 2701, a display 2702, a speaker 2703, a record medium 2704 and anoperation switch 2705. The recording medium may be a DVD (digitalversatile disk) and compact disk (CD) and used to play back musicprograms, display video images, play video games (or television games)and display information via Internet. This invention can be suitablyapplied to the display 2702 and signal control circuits.

FIG. 19(C) shows a digital camera, which includes a body 2801, a display2802, an eyepiece 2803, an operation switch 2804, and a picturereceiving section (not shown). This invention can be applied to thedisplay 2802 and signal control circuits.

FIG. 20(A) shows a front type projector, which includes a light sourceoptical system and display 2901 and a screen 2902. This invention can beapplied to the display and signal control circuits. FIG. 20(B) shows arear type projector, which includes a body 3001, a light source opticalsystem and display 3002, a mirror 3003, and a screen 3004. Thisinvention can be applied to the display and signal control circuits.

FIG. 20(C) shows one example of the constitution of the light sourceoptical system and display 2901, 3002 of FIGS. 20(A) and 20(B). Thelight source optical system and display 2901, 3002 includes a lightsource optical system 3101, mirrors 3102, 3104-3106, a dichroic mirror3103, a beam splitter 3107, a liquid crystal display 3108, a phase plate3109, and a projection optical system 3110. The projection opticalsystem 3110 comprises a plurality of optical lenses. Although FIG. 20(C)shows a three-plate type optical system that uses three liquid crystaldisplays 3108, the optical system is not limited to this type and asingle plate type optical system may be used. In the light pathsindicated by arrows in FIG. 20(C), optical lenses, polarizing films,phase adjusting films or IR films may be provided. FIG. 20(D) shows oneexample of the constitution of the light source optical system 3101 ofFIG. 20(C). In this example, the light source optical system 3101comprises a reflector 3111, a light source 3112, lens arrays 3113, 3114,a polarizing conversion element 3115, and a focusing lens 3116. Thelight source optical system shown in FIG. 20(D) is only one example andthe invention is not limited to this configuration.

This invention can also be applied to navigation systems (though notshown) and reading circuits of image sensors. As described above, theinvention has a very wide range of application covering electronicapparatus in every field. The electronic apparatus of this example canbe realized by any combination of this example and the precedingexamples 1-12. With this invention, even when the n-channel TFT in thepixel area is applied a gate voltage of 15 to 20 V, it can be operatedstably. This enhances the reliability of the semiconductor devicesincluding CMOS circuits comprising crystal TFTs and, more specifically,the reliability of the pixel area of the liquid crystal display and ofthe drive circuits provided at the periphery of the pixel area. Theliquid crystal display fabricated with this invention therefore canwithstand many hours of use.

1-25. (canceled)
 26. A semiconductor device comprising: a thin filmtransistor comprising: a semiconductor film having a pair of impurityregions and a channel region interposed therebetween, and a pair oflightly doped regions between the channel region and the pair ofimpurity regions; a gate insulating film formed over the semiconductorfilm; a gate electrode formed over the channel region with a gateinsulating film interposed therebetween, wherein the gate electrodecomprises a first conductive layer formed on the gate insulating film,and a second conductive layer formed on the first conductive film,wherein the second conductive layer extends beyond side edges of thefirst conductive layer, and contacts with the gate insulating film, andwherein an extended portion of the second conductive layer overlaps thepair of lightly doped regions partly, a planarizing film formed over thethin film transistor; a pixel electrode formed over the planarizingfilm, and electrically connected to one of the pair of impurity regions;and a light emitting layer over the pixel electrode.
 27. A semiconductordevice comprising: a plurality of thin film transistors connected inseries, each of the plurality of thin film transistors comprising: asemiconductor film having a pair of impurity regions and a channelregion interposed therebetween, and a pair of lightly doped regionsbetween the channel region and the pair of impurity regions; a gateinsulating film formed over the semiconductor film; a gate electrodeformed over the channel region with a gate insulating film interposedtherebetween, wherein the gate electrode comprises a first conductivelayer formed on the gate insulating film, and a second conductive layerformed on the first conductive film, wherein the second conductive layerextends beyond side edges of the first conductive layer, and contactswith the gate insulating film, and wherein an extended portion of thesecond conductive layer overlaps the pair of lightly doped regionspartly, a planarizing film formed over the thin film transistor; a pixelelectrode formed over the planarizing film, and electrically connectedto one of the pair of impurity regions; and a light emitting layer overthe pixel electrode.
 28. A semiconductor device comprising: a pluralityof thin film transistors connected in parallel, each of the plurality ofthin film transistors comprising: a semiconductor film having a pair ofimpurity regions and a channel region interposed therebetween, and apair of lightly doped regions between the channel region and the pair ofimpurity regions; a gate insulating film formed over the semiconductorfilm; a gate electrode formed over the channel region with a gateinsulating film interposed therebetween, wherein the gate electrodecomprises a first conductive layer formed on the gate insulating film,and a second conductive layer formed on the first conductive film,wherein the second conductive layer extends beyond side edges of thefirst conductive layer, and contacts with the gate insulating film, andwherein an extended portion of the second conductive layer overlaps thepair of lightly doped regions partly, a planarizing film formed over thethin film transistor; a pixel electrode formed over the planarizingfilm, and electrically connected to one of the pair of impurity regions;and a light emitting layer over the pixel electrode.
 29. A semiconductordevice according to claim 26, further comprising a bank formed over theplanarizing film, and connected to the light emitting layer.
 30. Asemiconductor device according to claim 27, further comprising a bankformed over the planarizing film, and connected to the light emittinglayer.
 31. A semiconductor device according to claim 28, furthercomprising a bank formed over the planarizing film, and connected to thelight emitting layer.
 32. A semiconductor device according to claim 26,further comprising a first passivation film formed over the thin filmtransistor.
 33. A semiconductor device according to claim 26, furthercomprising a first passivation film formed over the plurality of thinfilm transistors.
 34. A semiconductor device according to claim 26,further comprising a first passivation film formed over the plurality ofthin film transistors.
 35. A semiconductor device according to claim 26,further comprising a second passivation film formed over the lightemitting layer.
 36. A semiconductor device according to claim 27,further comprising a second passivation film formed over the lightemitting layer.
 37. A semiconductor device according to claim 28,further comprising a second passivation film formed over the lightemitting layer.
 38. A semiconductor device according to claim 26,wherein the semiconductor device is applied to semiconductor apparatusselected from the group consisting of a cellular phone, a video camera,a mobile computer, a head-mounted display, a rear type projector, aportable book, a personal computer, a player that uses a recordingmedium, a digital camera, and a front type projector.
 39. Asemiconductor device according to claim 27, wherein the semiconductordevice is applied to semiconductor apparatus selected from the groupconsisting of a cellular phone, a video camera, a mobile computer, ahead-mounted display, a rear type projector, a portable book, a personalcomputer, a player that uses a recording medium, a digital camera, and afront type projector.
 40. A semiconductor device according to claim 28,wherein the semiconductor device is applied to semiconductor apparatusselected from the group consisting of a cellular phone, a video camera,a mobile computer, a head-mounted display, a rear type projector, aportable book, a personal computer, a player that uses a recordingmedium, a digital camera, and a front type projector.